1. Field of the Invention
This invention relates to a method and an apparatus for measuring the eccentricity of the tracks of a disk such as a CD or an MD and rating the disk for the quality.
2. Related Background Art
Generally, disks such as CDs and MDs are measured for the eccentricity of the tracks and rated in terms of the detected eccentricity of the tracks.
There are two known techniques for measuring the eccentricity of the tracks of an optical disk such as a CD or an MD by means of a player type instrument.
With one of the known techniques, the light beam from an optical pickup is made to produce a stationary light spot and the optical disk is rigidly held to a turn table arranged on a spindle and turned by means of the turn table, while the tracking servo is held off.
With this arrangement, the light beam crosses tracks within a full turn of the optical disk, the number of which is proportional to the eccentricity of the tracks as shown in FIG. 1. Note that the tracks move back and forth by a distance twice as much as the eccentricity within a full turn of the disk as shown in FIG. 2. In FIGS. 1 and 2, reference numeral 1 denotes an optical disk and reference symbols 1a, 1b and 1c respectively denote the positioning hole of the disk operating as center of rotation, the center of the tracks and the eccentricity of the tracks.
Thus, the light beam crosses a number of tracks that corresponds to four times of the eccentricity within a full turn of the disk. If the track pitch is 1.60 xcexcm and the eccentricity of the tracks is 16.0 xcexcm, the light beam crosses a total of 16.0 xcexcmxc3x974÷1.60 xcexcm=40 tracks while the disk makes a full turn.
When the tracking error signal is observed during this measuring operation, it will be seen that the wave of the signal moves up and down (a motion referred to as traverse) once each time the light beam crosses a track. With this technique, the optical disk 1 is made to turn a plurality of times and the number of traverses is counted. Then, the number of traverses per turn and hence the eccentricity of the optical disk can be determined.
However, this measuring technique has a drawback that, if the spindle itself that drives the optical disk to turn involves eccentricity, the measured eccentricity includes that of the spindle and hence there is no way of knowing the true quantity of eccentricity of the optical disk 1.
With the other known measuring technique, the optical disk is played ordinarily while the tracking servo is held on. Then, if the tracks are arranged eccentrically, they will always be swerved radially so that the light beam is servo-controlled to follow the swerving motion of the tracks. Then, it will be found by observing the tracking error signal or the drive signal of the tracking servo that it meanders as shown in FIG. 3 as the spindle turns to drive the optical disk.
The meandering of the tracking error signal represents the swerving motion of the tracks that is twice as much as the eccentricity of the tracks even in a single turn of the optical disk so that the eccentricity is expressed by a quantity corresponding to xc2xd of the amplitude of the signal.
However, this second measuring technique also has the drawback that, if the spindle itself that causes the optical disk to turn involves eccentricity, the measured eccentricity includes that of the spindle and hence there is no way of knowing the true quantity of eccentricity of the optical disk 1. Furthermore this second measuring technique has an additional drawback that the relationship between the extent of the meandering of the electric signal as shown in FIG. 3 and the extent of the movement of the light beam has to be defined.
It is the object of the present invention to provide a method and an apparatus that can measure the true quantity of eccentricity of the tracks of a disk with a relatively simple arrangement.
According to the invention, the above object is achieved by providing a method of measuring the eccentricity of the tracks of a disk comprising steps of:
rigidly securing the disk to be evaluated to a turn table arranged on a spindle and driving the disk to reproduce the information recorded thereon by means of a reproducing head;
determining the extent of eccentricity from the signal reproduced by said reproducing head and the direction of eccentricity from the tracking error signal and the angle of rotation of said spindle obtained by turning on the tracking servo; and
subsequently determining the quantity of eccentricity of the tracks of the disk to be evaluated in terms of vector quantity on the basis of said extent of eccentricity and said direction of eccentricity and then subtracting the vector quantity of the eccentricity of said spindle from the vector quantity of the eccentricity of the tracks of the disk.
Thus, according to the invention, the eccentricity of the tracks of the disk to be evaluated that is rigidly secured to a turn table arranged on a spindle is determined in therms of vector quantity and the vector quantity of the eccentricity of the spindle is subtracted from the vector quantity of the eccentricity of the tracks of the disk so that it is possible to determine the true quantity of eccentricity of the tracks of the disk to be evaluated that is free from the eccentricity of the spindle.
In another aspect of the invention, there is also provided a method of measuring the eccentricity of the tracks of a disk comprising steps of:
rigidly securing the disk to be evaluated to a turn table arranged on a spindle and driving the disk to reproduce the information recorded thereon by means of a reproducing head;
determining the first extent of eccentricity from the signal reproduced by said reproducing head and the first direction of eccentricity from the tracking error signal and the angle of rotation of said spindle obtained by turning on the tracking servo and subsequently determining the eccentricity of the tracks of the disk in terms of vector quantity from said first extent of eccentricity and said first direction of eccentricity;
rigidly securing said disk to said turn table so as to shift the phasic relationship of said disk and said spindle by 180xc2x0;
determining the second extent of eccentricity from the signal reproduced by said reproducing head and the second direction of eccentricity from the tracking error signal and the angle of rotation of said spindle obtained by turning on the tracking servo and subsequently determining the eccentricity of the tracks of the disk in terms of vector quantity from said second extent of eccentricity and said second direction of eccentricity;
subsequently determining the true quantity of eccentricity of the tracks of the disk to be evaluated by subtracting the second vector quantity from the first vector quantity.
With the above arrangement, since the second vector quantity of the eccentricity of the tracks of the disk to be evaluated is determined by shifting the phasic relationship between the disk and the spindle by 180xc2x0, if the first vector quantity E1 is expressed by formula below:
vector E1=vector ME+vector E,
where ME is the vector quantity of the eccentricity of the spindle and E is the true vector quantity of eccentricity of the tracks of the disk to be evaluated, then the second vector quantity E2 is expressed by formula below:
vector E2=vector MExe2x88x92vector E.
Thus, the true quantity of eccentricity E of the tracks of the disk to be evaluated is obtained by subtracting the second vector quantity E2 from the first vector quantity E1 or
vector E1xe2x88x92vector E2=2xc3x97vector E.
In still another aspect of the invention, there is also provided an apparatus of measuring the eccentricity of the tracks of a disk comprising:
a spindle for driving a turn table adapted to carry a disk to be evaluated;
angle of rotation detecting means for detecting the angle of rotation of the spindle;
a reproducing head for reproducing the information stored on the disk;
tracking servos for control the tracking operation of the reproducing head;
extent of eccentricity measuring means for measuring the extent of eccentricity from the signal reproduced by the reproducing head;
tracking error detecting means for obtaining a tracking error signal by turning on the tracking servo; and
an operation means for determining the direction of eccentricity from the angle of rotation of the spindle and the tracking error signal, determining the vector quantity of the eccentricity of the tracks of the disk from the extent of eccentricity and the direction of eccentricity and carrying out a vector operation to determine the true quantity of eccentricity of the tracks of the disk.
Thus, according to the invention, it is now possible to determine the true quantity of eccentricity of the tracks of a disk to be evaluated that does not include the eccentricity of the spindle by means of a relatively simple arrangement.